Confused? So was the National Research Council, the nonprofit face of the expert panels on science, engineering and medicine created by Congress to provide sound advice on the thorny issues of our times.

The council decided that -- 35 years after the U.S. Supreme Court ended a four-year moratorium on capital punishment in the United States -- now was the time to review the scientific research to see if it was possible to answer whether the death penalty deters would-be killers.

The answer, made public last week in a 144-page book, is: no.

“Fundamental flaws in the research we reviewed make it of no use in answering the question of whether the death penalty affects homicide rates,” wrote Daniel S. Nagin, a professor of public policy and statistics at Carnegie Mellon University in Pittsburgh, who chaired the blue-ribbon committee – it included James Q. Wilson, the late political scientist and expert on crime and society -- that wrote the report.

The report found fundamental faults in all the studies purporting to answer the deterrence question.

Some studies simply assumed what potential murderers think about the abstract possibility of the death penalty despite the fact that only 15 percent of inmates condemned since 1976 were actually put to death and many death sentences were reversed.

The studies were also based on the assumption that public and official attitudes toward capital punishment were the same nationwide, rather than varying among the states and over time.

“There is no evidence to support such suppositions,” the report found.

Nor did any of the research consider the possible deterrent impact of life prison terms or other forms of non-capital punishment on homicide rates, the report continued.

“These intrinsic shortcomings severely limit what can be learned from the existing research,” the report concluded.

Although the report’s authors said they believed it possible to scientifically determine if the death penalty affects the homicide rate, they said credible results will depend on using statistical methods to compare the deterrence of capital and non-capital punishment and gauge how potential murderers perceive the possible penalties they would face.

“Some people may find partial conclusions unappealing and may be tempted to impose strong assumptions in order to obtain definitive answers,” the report concludes. “We caution against this reaction. Imposing strong but untenable assumptions cannot truly resolve inferential problems. Rather, it simply replaces the modeling uncertainty with uncertainty associated with the underlying assumptions.”

“Today, more than 30 years later,” the report continues, “perhaps the primary lesson learned from the latest round of empirical research on the deterrent effect of the death penalty is that research and policy makers must cope with ambiguity. Explicitly recognizing and accounting for this uncertainty seems like the only hope of moving forward.”